Aymenn Jawad Al-Tamimi in the Media

Syrian opposition fractures

Syrian rebel groups have begun fierce fighting against the Islamic State of Iraq and al- Sham (ISIS) across the north of the country.

Often described as al-Qa'ida, ISIS is not officially linked to the group, but is closely affiliated and shares ideology with the terrorist group. Speaking in Israel this week, the Secretary of State John Kerry, described ISIS as "the most dangerous players in this region."

Widespread anger at the repressive and arbitrary nature of ISIS's methods has been growing since the groups' arrival in northern Syria in May 2013. Many of the Islamist groups want the formation of an Islamic state in Syria. But ISIS goes even further, calling for the restoration of the caliphate, an Islamic kingdom, across the Levant. ISIS members have gained a reputation for brutal punishments for failure to comply with their strict interpretation of Islamic law and are often criticised for their focus on governance and state building in the areas they occupy, rather than fighting against the government of Bashar al-Assad alongside other rebel groups. The backlash against the group was finally triggered by the killing of a popular Ahrar al-Sham leader, Dr Hussein al-Suleiman, and attacks by the group within the town of Kafranbel. Ahrar al-Sham is Islamist but is not affiliated with al-Qa'ida. It is a major player in the consolidation of rebel groups called the Islamic Front (IF). The front was formed late last year and was quick to reject the Syrian National Coalition (SNC) and their military leader Salam Idriss, claiming the group, which is closely tied to western powers, lacks influence on the ground inside Syria.

Dr Hussein, a popular commander, had gone to negotiate with ISIS about a local dispute in Maskanah, near Aleppo. His body was returned with an ear missing and visible signs of torture, and his brutal killing sparked protests. In scenes reminiscent of the early days of the revolution against Assad, ISIS fired on protestors, igniting further anger. A cousin of Dr Hussein, who spoke on condition of anonymity, was keen for accountability and told The Media Line: "Now is the time for action against ISIS."

In Kafranbel, Muhammad Khatib, an activist from the town told The Media Line that "Daesh [a local name for ISIS] tried to impose their ideology on the Kafranbel people. They kidnapped activists and destroyed the media centre. They banned smoking, demanded Islamic attire for all the girls. They tried to ban jeans and casual clothes, and asked men to grow beards. The people were fed up in less than a month."

The Islamic Front was quick to condemm the killing of Dr Hussein. Hassan Aboud, the political leader of the group, told Al Jazeera: "ISIS denies reality, refusing to recognize that it is simply another group. It refuses to go to independent courts; it attacked many other groups, stole their weapons, occupied their headquarters, and arbitrarily apprehended numerous activists, journalists and rebels. It has been torturing its prisoners. These transgressions accumulated, and people got fed up with ISIS. Some of those people have attacked ISIS's positions, but ISIS was first to attack in other places, bringing this on itself."

Military action from the Islamic Front and two other rebel coalitions, the Syrian Revolutionary Front (SRF) and Jaysh al-Mujahideen (a new formation of 7-Aleppo based brigades), was quick to follow the condemnation. "FSA in Kafranbel and Maarat Alnouman attacked many ISIS holds in the region and released 14 Syrian peaceful activists and arrested many of the ISIS," Mohammed Khatib, an activist in Kafranbel told The Media Line. "The big camp of ISIS near Kafranbel was given 24 hours notice to leave, and they did leave."

In Aratib, in the outskirts of Aleppo, the Islamic Front and ISIS engaged in fierce battles which resulted in the death of the Emir, or leader of ISIS for the area. As the three rebel coalitions seized on the opportunity created to attack ISIS positions throughout the Aleppo region and Idlib, the fighting spread to other towns where tensions have been bubbling for months, including the border town of Azaz, the scene of previous battles between ISIS and IF brigades. Dozens of fighters on each side have been killed and many more injured, although the figures released by the Syrian Observatory for Human Rights suggest the losses are heavier within the FSA affiliated groups. There have also been reports of prisoners being killed en masse by ISIS fighters as they abandon positions or defect to the rebels.

Aymenn Jawad Al-Tamimi, Shillman-Ginsburg Fellow at the Middle East Forum, says the clashes are not part of a coordinated operation.

"IF operations are not coordinated or pre-planned here," he said. "These clashes with ISIS are spontaneous and localized."

Syrian activists have been quick to call this the 'second revolution', but the Islamic front have left the door open for a negotiated peace with ISIS.

"We would like these [ISIS] brothers to join their brethren in the Syrian revolution," Hassan Aboud of the Islamic Front said. "We see them as nothing but another group.

They see themselves as a state. They need to drop this illusion that they have come to believe as an established fact. It causes them to treat allies as opponents."

If ISIS did join the Islamic Front, this would not be viewed as good news by the United States, especially ahead of the planned Geneva II peace talks seeking an end to the war in Syria. Widely viewed as a terror group by governments outside of Syria, absorption of them into rebel forces would further intensify the ideological differences between western powers and the Islamic Front. The Islamic extremist group yesterday claimed responsibility for a bombing in a Hizbullah area of Beirut and took control of the city of Falluja in Iraq last week, prompting Kerry's comments that "The rise of these terrorists in the region, and particularly in Syria, and through the fighting in Syria, is part of what is unleashing this instability in the rest of the region. That's why everybody has a stake."

While the fighting is more widespread than previous clashes among the rebel groups, it is not clear if the differences can be settled without further escalation.

"If ISIS carries through with its threat to withdraw from key Aleppo theatres, fighting will almost certainly escalate further," Charles Lister, a visiting fellow at Brookings Doha Center told The Media Line. "If corroborated, reports this morning of ISIS car bombs targeting rebel checkpoints as part of a limited counter-attack would be a bad sign."